A drive assisting device that is mounted on a vehicle to assist the traveling of the vehicle is conventionally known. The drive assisting device determines the behavior of the vehicle based on various properties of the vehicle to assist the traveling of the vehicle. The various properties of the vehicle include properties that fluctuate according to the conditions at the time of traveling. A device for detecting the properties of the vehicle that fluctuate includes, for example, a device described in Patent Literature 1. Patent Literature 1 describes a device that estimates the weight of the vehicle. Patent Literature 1 also describes prohibiting the computation of the weight of the vehicle (estimation of the vehicle weight) when the vehicle speed exceeds a predetermined vehicle speed.